Reflective photomask and method of fabricating the same

ABSTRACT

A reflective photomask comprises a photomask substrate, a photomask pattern, formed on an upper surface of the photomask substrate, at least one alignment mark, formed on the upper surface of the photomask substrate, for aligning the reflective photomask with an exposure apparatus, and at least one fiducial mark, formed on a lower surface of the photomask substrate, for determining locations of defects in the photomask pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. non-provisional patent application claims priority under 35U.S.C. §119 of Korean Patent Application 10-2009-0022311, filed on Mar.16, 2009, the entire contents of which are hereby incorporated byreference in their entirety.

BACKGROUND

The present disclosure herein relates to a reflective photomask and amethod of fabricating the same.

There has been an increasing demand for forming smaller patterns onsemiconductor substrates. In order to satisfy this demand, thewavelength of a light source, which is used to form patterns onsemiconductor devices in a lithography step, has become shorter. Forexample, in the past, the lithography step utilized light having ag-line wavelength band (e.g., approximately 436 nm) or an i-linewavelength band (e.g., approximately 365 nm). Use of light having adeep-ultraviolet wavelength band is becoming more prevalent. Moreover,the lithography step will likely utilize light having anextreme-ultraviolet (EUV) wavelength band in the future.

Because light having an EUV wavelength band is mostly absorbed byrefractive optical materials, EUV lithography utilizes a reflectiveoptical system instead of a refractive optical system. Thus, EUVlithography requires a reflective photomask in which circuit patterns tobe transferred onto a wafer are formed on a reflective surface. This EUVphotomask may include a plurality of layers. Various types of defectsmay occur when forming a plurality of layers on the EUV photomask. Itmay be difficult or impossible to completely remove these defects. Thus,the cost of fabricating EUV photomasks is greatly affected by defectcontrol.

Therefore, a need exists for a reflective photomask and for a method ofcontrolling defects during the fabrication of the reflective photomask

SUMMARY

The present disclosure is to provide a method of fabricating an extremeultraviolet photomask that can maintain the accuracy of locationinformation for defects.

The present disclosure is to provide a photomask that can preciselyapply a defect avoidance technique.

According to an exemplary embodiment of the inventive concept, areflective photomask comprises a photomask substrate, a photomaskpattern, formed on an upper surface of the photomask substrate, at leastone alignment mark, formed on the upper surface of the photomasksubstrate, for aligning the reflective photomask with an exposureapparatus, and at least one fiducial mark, formed on a lower surface ofthe photomask substrate, for determining locations of defects in thephotomask pattern.

In another exemplary embodiment, the photomask pattern comprises aplurality of thin layers.

In another exemplary embodiment, the plurality of thin layers comprisesat least one of a multilayer, a capping layer, a buffer layer, anabsorbing layer, and a photoresist layer.

In another exemplary embodiment, the plurality of thin layers comprisesalternating molybdenum layers and silicon layers.

In another exemplary embodiment, the photomask pattern and the alignmentmark are formed on a top layer of the upper surface and the fiducialmark is formed on a bottom layer of the lower surface.

In another exemplary embodiment, the photomask substrate comprises amaterial having a low thermal expansion property, such as, for example,glass.

In another exemplary embodiment, the reflective photomask comprises aconductive layer for fixing the reflective photomask to an exposureapparatus via an electrostatic chucking effect.

In another exemplary embodiment, the plurality of thin layers forms aBragg reflector.

According to an exemplary embodiment of the inventive concept, a methodof fabricating a reflective photomask comprises forming at least onefiducial mark on a lower surface of a photomask substrate, forming aplurality of thin layers on an upper surface of the photomask substrate,inspecting the plurality of thin layers for defects, and extractingdefect data corresponding to the plurality of thin layers, wherein thedefect data comprises location information corresponding to defectsformed in the plurality of thin layers.

In another exemplary embodiment, the location information uses alocation of the fiducial mark as a reference point.

In another exemplary embodiment, the method comprises aligning a loweroptical system with the fiducial mark, inspecting the plurality of thinlayers for defects using an upper optical system, determining locationsof the defects relative to the fiducial mark based on a position of theupper optical system relative to a position of the lower optical system,and storing the locations of the defects as the defect data.

In another exemplary embodiment, the method comprises aligning a loweroptical system with an upper optical system, inspecting the plurality ofthin layers for defects using the upper optical system, determininglocations of the defects relative to the fiducial mark based on aposition of the upper optical system relative to a position of the loweroptical system, and storing the locations of the defects as the defectdata, wherein the positions of the upper optical system and the loweroptical system are fixed.

In another exemplary embodiment, the method comprises preparing a defectavoidance layout based on defect avoidance data, wherein the defectavoidance data is obtained by comparing the defect data with photomaskpattern data, and the defect avoidance layout minimizes an overlapbetween circuit patterns and the defects, and patterning the defectavoidance layout onto the thin layers on the upper surface of thephotomask substrate.

In another exemplary embodiment, the method comprises determining aposition of the fiducial mark using a lower optical system, wherein thedefect avoidance layout is provided as a relative coordinate withrespect to the fiducial mark.

In another exemplary embodiment, patterning the thin layers comprisesaligning a lower optical system with an exposure system and irradiatinga light source onto a photoresist layer of the thin layers.

In another exemplary embodiment, the light source is irradiated onto apredetermined region of the photoresist layer based on defect avoidancedata.

In another exemplary embodiment, patterning the layers comprisesaligning a lower optical system with the fiducial mark and irradiating alight source onto a photoresist layer of the thin layers.

In another exemplary embodiment, the light source is irradiated onto apredetermined region of the photoresist layer based on defect avoidancedata.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the inventive concept, and are incorporated in andconstitute a part of this specification. The drawings illustrateexemplary embodiments of the inventive concept and, together with thedescription, serve to explain principles of the inventive concept. Inthe drawings:

FIG. 1 is a flowchart illustrating a step of fabricating and applying ageneral photomask according to an exemplary embodiment of the inventiveconcept;

FIG. 2 is a perspective view illustrating a reflective photomask blankaccording to an exemplary embodiment of the inventive concept;

FIGS. 3A and 3B are plan views illustrating a reflective photomaskaccording to an exemplary embodiment of the inventive concept;

FIG. 4 is a flowchart illustrating a step of fabricating a photomaskaccording to an exemplary embodiment of the inventive concept;

FIG. 5 is a flowchart schematically illustrating a step of inspecting aphotomask blank according to an exemplary embodiment of the inventiveconcept;

FIGS. 6 through 9 are diagrams illustrating a step of inspecting aphotomask blank according to an exemplary embodiment of the inventiveconcept;

FIG. 10 is a flowchart schematically illustrating a step of patterning aphotomask blank used to form a photomask according to an exemplaryembodiment of the inventive concept;

FIGS. 11 and 12 are diagrams illustrating an alignment between anexposure system and a lower optical system according to an exemplaryembodiment of the inventive concept;

FIG. 13 is a plan view illustrating a reflective photomask according toan exemplary embodiment of the inventive concept; and

FIGS. 14 and 15 are diagrams illustrating a stage according to anexemplary embodiment of the inventive concept.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Advantages and features of the present invention and methods ofaccomplishing the same may be understood more readily by reference tothe following detailed description of preferred embodiments and theaccompanying drawings. The exemplary embodiments of the inventiveconcept may, however, be embodied in many different forms and should notbe construed as being limited to the embodiments set forth herein.Rather, these embodiments are provided so that this disclosure will bethorough and complete and will fully convey the scope of the inventiveconcept to those skilled in the art, and the embodiments of theinventive concept will only be defined by the appended claims.

It will be understood that when any layers such as a conductive layer, asemiconductor layer, and an insulating layer are referred to as being“on” another material layer or substrate, it may be directly on theother material layer or substrate or intervening elements or layers maybe present. In drawings, the thickness of layers and regions isexaggerated to effectively describe technical details. It will beunderstood that, although the terms first, second, etc. are used hereinto describe various elements, these elements should not be limited bythese terms. These terms are used to distinguish one element fromanother. Accordingly, a layer being referred to as a first layer in oneembodiment may be referred to as a second layer in another embodiment.Each embodiment described and illustrated herein includes acomplementary embodiment thereof.

FIG. 1 is a flowchart illustrating a step of fabricating and applying ageneral photomask, according to an embodiment of the inventive concept.

Referring to FIG. 1, the steps of fabricating the photomask include thestep of preparing a photomask blank (S1) and forming photomask patterns(e.g., circuit patterns and prototype patterns)—which will betransferred onto a wafer—by processing the photomask blank

(S2). The step of applying the photomask includes the step of installingthe fabricated photomask in an exposure apparatus (S3) and transferringthe photomask patterns onto the wafer (S4).

FIG. 2 is a perspective view illustrating a reflective photomask blankaccording to an embodiment of the inventive concept.

Referring to FIG. 2, the reflective photomask blank according to anexemplary embodiment of the inventive concept may include a photomasksubstrate 10 (hereinafter referred to as a “substrate”), thin layers 20formed on an upper surface of the substrate 10, and at least onefiducial mark FM formed on a lower surface of the substrate 10. The thinlayers 20 may include at least one of a multilayer 11, a capping layer12, a buffer layer 13, an absorbing layer 14, and a mask layer 15 whichmay be sequentially stacked, as illustrated in FIG. 2.

The substrate 10 may be formed of a material having a low thermalexpansion property (e.g., glass). In order to improve the reflectance ofan extreme ultraviolet (EUV) light used in EUV exposure system, themultilayer 11 may be comprised of a plurality of thin layers, forming aBragg reflector. In a Bragg reflector, the boundary between each of theplurality of layers causes a partial reflection of a light sourceirradiating the multilayer 11. The multilayer 11 may compriseapproximately 40 to 60 thin layers. According to an exemplary embodimentof the inventive concept, the multilayer 11 may include molybdenumlayers and silicon layers which are alternately stacked. For example,each of the molybdenum layers may be substantially formed to have athickness of approximately 2.8 nm, and each of the silicon layers may besubstantially formed to have a thickness of approximately 4.0 nm or 4.1nm. These values are for exemplary purposes, and the thickness of thethin layers may be set to different values in consideration of thewavelength of the EUV light to be used.

The absorbing layer 14 may be formed of any one of materials capable ofabsorbing the EUV light. According to an exemplary embodiment of theinventive concept, the absorbing layer 14 may be formed of a conductiveabsorber such as, for example, tantalum nitride (TaN). However, thematerial used for the absorbing layer 14 is not limited to theexemplified tantalum nitride. The buffer layer 13 or the capping layer12 may be used as an etch stop layer in an etching step of patterningthe absorbing layer 14. According to an exemplary embodiment of theinventive concept, the buffer layer 13 or the capping layer 12 maycomprise, for example, a silicon nitride layer or a silicon oxide layer.According to another exemplary embodiment of the inventive concept, theabsorbing layer 14 may be directly formed on the multilayer 11 withoutthe presence of the buffer layer 13 or the capping layer 12. The masklayer 15 may be a photoresist layer.

According to an exemplary embodiment of the inventive concept, aconductive layer 30 may be formed on the lower surface of the substrate10 so as to define shapes or locations of the fiducial mark(s) FM. Thepresence of the conductive layer 30 allows an electrostatic chuckingeffect to be used to fix the photomask to the exposure apparatus. Thefiducial mark(s) FM may be formed on the lower surface of the substrate10 regardless of the existence of the conductive layer 30.

The fiducial mark(s) FM is used as a reference point for describing thelocations of defects which may occur during formation of the thin layers20. When the fiducial mark(s) FM is formed on the upper surface of thesubstrate 10, the shape of the fiducial mark(s) FM may gradually bedistorted as the thickness or the number of the thin layers 20 stackedon the upper portion of the fiducial mark(s) FM increases. Thisdistortion may result in decreased precision of the detected locationsof the defects. When the fiducial mark(s) FM is formed on the lowersurface of the substrate 10, as described above, the fiducial mark(s) FMis not influenced by the formation of the thin layers 20 or by thenumber or thickness of the layers. As a result, the function of thefiducial mark(s) FM as a spatial reference point does not deteriorate.

The fiducial mark(s) FM may be formed on at least one or all of thecorners of the substrate 10 as illustrated in FIGS. 2 and 3A.Alternatively, the fiducial mark(s) FM may be formed at any positiondistant from edges of the substrate 10 as illustrated in FIG. 13.

FIGS. 3A and 3B are plan views illustrating the reflective photomaskaccording to an exemplary embodiment of the inventive concept. Thereflective photomask illustrated in FIGS. 3A and 3B may be formed bypatterning the photomask blank of FIG. 2

Referring to the exemplary embodiments illustrated in FIGS. 3A and 3B,the photomask may include photomask patterns and a fiducial mark(s) FMformed on the upper and lower surfaces of the substrate 10,respectively. Alignment marks for the alignment between the photomaskand the exposure apparatus, and auxiliary patterns for other variousfunctions may be further disposed on the upper surface of the substrate10, as illustrated in FIG. 3B.

The photomask patterns, the alignment marks, and the auxiliary patternsmay be obtained by patterning the thin layers 20 formed on the uppersurface of the substrate 10. The step of patterning the thin layers 20will be described in detail below with reference to FIG. 4.

The alignment marks formed on the upper surface of the substrate 10 aredistinguished from the fiducial mark(s) FM formed on the lower surfaceof the substrate 10 because the alignment marks are used to align thephotomask and the exposure apparatus and the fiducial mark(s) FM areused to detect the locations of defects which may occur during the stepof forming the thin layers 20. Because the alignment marks are formed bypatterning the thin layers 20, the alignment marks may not be used asfiducial marks FM.

According to another exemplary embodiment of the inventive concept, thefiducial marks FM are formed before the thin layers 20 are patterned andare not removed subsequent to the fabrication of the photomask. In thiscase, the fiducial marks FM may be used as alignment marks wheninstalling the photomask in the exposure apparatus.

Further, the photomask patterns may include inner alignment patterns(not illustrated) used for the alignment between the patterns formed onthe wafer. The inner alignment patterns may be formed in a scribe laneregion between chip regions. Because the inner alignment patterns areformed after the thin layers 20 of the photomask blank are patterned,the inner alignment patterns are not used for the step of patterning thethin layers 20.

FIG. 4 is a flowchart illustrating more fully the step of fabricatingthe photomask according to an exemplary embodiment of the inventiveconcept.

Referring to FIGS. 1 and 4, the step S1 of preparing the photomask blankmay include steps of preparing the photomask substrate 10 (S11), formingthe fiducial mark(s) FM on the lower surface of the photomask substrate10 (S12), and forming the thin layers 20 on the upper surface of thephotomask substrate 10 (S14).

After forming the thin layers 20, the locations of any defects, whichmay occur in the step of forming the thin layers, may be inspected(S15). In this step, any number of the thin layers 20 may be inspected.For example, if necessary, inspection of certain thin layers 20 may beomitted. The location of any defects found during step S15 is stored asdefect data D2. According to an exemplary embodiment of the inventiveconcept, the fiducial mark(s) FM may be used as a common reference fordefining the location information of the defects occurring in the stepof forming each of the thin layers 20. When the fiducial marks FM areformed on the lower surface of the substrate 10, as described above,high precision of the detected location of the defects may bemaintained, regardless of the thickness and/or number of layerscomprising the thin layers 20.

According to another exemplary embodiment of the inventive concept, thestep S13 of inspecting the photomask substrate 10 may be performedbefore the thin layers 20 are formed. The location information of anydefects discovered during the inspection in Step S13 may be added to thedefect data D2.

The step S12 of forming the fiducial marks FM may comprise disposing amold for defining the fiducial marks FM on the lower surface of thesubstrate 10 and coating the conductive layer 30 on the periphery of themold. In this case, the fiducial marks FM may be obtained by removingthe mold. According to the exemplary embodiment, the mold may be used asa clamping member for fixing the substrate 10 during the step of coatingthe conductive layer 30 (e.g., by clamping the corners of the lowersurface of the substrate 10). In this case, as illustrated in FIG. 2,the fiducial marks FM are formed at the corners of the lower surface ofthe substrate 10. According to another exemplary embodiment of theinventive concept, the step S12 of forming the fiducial marks FM mayfurther include forming the conductive layer 30 on the entire lowersurface of the substrate 10 and patterning the conductive layer 30. Inthis case, the location of the fiducial marks FM and the shape of thefiducial marks FM may be freely selected.

The step S2 of processing the photomask blank may include creatingdefect avoidance data D3, which may be used during the patterning of thephotomask blank. The defect avoidance data D3 may be prepared by usingphotomask pattern data D1, which corresponds to the type of photomaskbeing used, and/or the defect data D2, which is created during the stepS1 of preparing the photomask blank.

Photomask pattern data D1 may be used to provide information on theposition of the circuit patterns (e.g., the photomask patterns) to betransferred onto the wafer. Photomask pattern data D1 may be provided ascoordinate information with respect to a predetermined reference point(hereinafter referred to as a “design reference point”). As describedabove, defect data D2 includes the coordinate information of the defectsdetermined by using the fiducial mark(s) FM as a reference point(hereinafter referred to as a “blank reference point”). Defect avoidancedata D3 can provide information (hereinafter referred to as “avoidanceinformation”) regarding how the design reference point corresponds tothe blank reference point. Thus, the avoidance data D3 may be used tominimize overlap between the circuit patterns defined in the photomaskpattern data D1 and the defects defined in the defect data D2. Forexample, overlap between the circuit patterns and the defects may beminimized by comparing the defect data with photomask pattern data andpreparing a defect avoidance layout to be patterned onto the thin layers20 of the substrate 10. Avoidance information may be obtained, forexample, by using the Numerical Analysis method using a computer such asa simulation, and the avoidance information may be recorded as arelative coordinate between the design reference point and the blankreference point.

Minimizing the overlap between the circuit patterns and the defectsprevents defects from being transferred onto the wafer. Because it maybe difficult to completely remove defects from the photomask, thedefects may remain on the photomask and be transferred onto the wafer.With the use of the defect avoidance data, however, the defects may beprevented from being transferred onto the wafer.

FIG. 5 is a flowchart schematically illustrating the step of inspectingthe photomask blank according to an exemplary embodiment of theinventive concept. FIGS. 6 through 8 are diagrams illustrating the stepof inspecting the photomask blank according to an exemplary embodimentof the inventive concept. The step of inspecting the photomask blank,which will be described in more detail below, may be performed on eachof the thin layers 20 and the photomask substrate 10, as illustratedwith reference to FIG. 4.

Referring to FIG. 6, facilities for inspecting the photomask blank willbriefly be described. The facilities may include a stage ST in which thephotomask blank PB, provided with the fiducial marks FM, is disposed.The facilities may further include an upper optical system 300 locatedabove the stage ST, and a lower optical system 200 located below thestage ST. The upper optical system 300 is configured to inspect thedefects generated in the photomask blank PB, and the lower opticalsystem 200 is configured to be aligned with the fiducial mark(s) FM.

Referring to FIGS. 5 through 8, the step S13 or S15 of inspecting thephotomask blank, as referred to in FIG. 4, may include: aligning thelower optical system 200 to the upper optical system 300 (S21), asillustrated in FIG. 6; aligning the lower optical system 200 to thefiducial marks FM (S22), as illustrated in FIG. 7; inspecting thedefects formed on the photomask blank PB by using the upper opticalsystem 300 (S23), as illustrated in FIG. 7; and storing the coordinatesof the defects as the defect data (S24).

In step S21, as referred to in FIG. 5, the lower optical system 200 maybe aligned with the upper optical system 300, as illustrated in FIG. 6.Once aligned, the relative position between the lower optical system 200and the upper optical system 300 may remain fixed.

When the lower optical system 200 is aligned with the fiducial marks FMin step

S22, as illustrated in FIG. 7, the defects inspected by the upperoptical system 300 in the step S23 are recorded as a relative positionwith respect to the fiducial marks FM, and are stored as the defect dataD2 in the step S24. The step S23 may further include analyzing anemitted light 302, which is emitted from the upper surface of thephotomask blank PB and is detected and analyzed by the upper opticalsystem 300, as illustrated in FIG. 8. The emitted light 302 is a resultof the optical interaction between the defects and the light incidentfrom the upper optical system 300.

FIG. 9 is a diagram illustrating the alignment between the upper opticalsystem 300 and the lower optical system 200 according to anotherexemplary embodiment of the inventive concept.

Referring to FIG. 9, a predetermined system alignment pattern 150 may bedisposed on one side of the stage ST. Optically discernible upper andlower marks may be formed on the top surface and the bottom surface ofthe system alignment pattern 150, respectively, and the upper and lowermarks may be vertically aligned. In this case, the lower optical system200 and the upper optical system 300 may be aligned with the lower andupper marks, respectively.

According to an exemplary embodiment of the inventive concept, the loweroptical system 200 or the upper optical system 300 may be configured todetect reflected light from a predetermined target. However, becauselenses comprising the lower optical system 200 and the upper opticalsystem 300 are formed of opaque materials through which reflected lightis difficult to emit, it may be difficult to align the lower opticalsystem 200 and the upper optical system 300 with each other through themanner illustrated in FIG. 6. According to another exemplary embodimentof the inventive concept, as illustrated in FIG. 9, the lower opticalsystem 200 and the upper optical system 300 may be aligned with a systemalignment pattern 150, since the system alignment pattern 150 may beused as a common target for the alignment between the lower opticalsystem 200 and the upper optical system 300. In an exemplary embodimentof the inventive concept, the system alignment pattern 150 may be formedat a fixed position with respect to the stage ST or the fiducial mark(s)FM.

FIG. 10 is a flowchart schematically illustrating the step of patterningthe photomask blank used to form the photomask according to an exemplaryembodiment of the inventive concept.

In the steps S13 and S15 of inspecting the photomask blank, as shown inFIG. 4, the upper optical system 300 is used as means for inspecting thedefects. In the step S16 of patterning the photomask blank, means (e.g.,an exposure system) for locally supplying energy to the photoresistlayer 15 in order to pattern the thin layers 20 is utilized. Forexample, instead of the upper optical system 300 illustrated withreference to FIG. 6, the exposure system 400 may be located above thestage ST, as illustrated in FIG. 11. The exposure system 400 may beconfigured to emit, for example, a light source having a narrow fluxsectional area onto the photoresist layer 15, however, the light sourceemitted by the exposure system 400 is not limited thereto.

Referring to FIG. 10, the step of patterning the photomask blank mayinclude: aligning the lower optical system 200 with the exposure system400 (S31); aligning the lower optical system 200 with the fiducialmark(s) FM (S32); and irradiating a light source onto the photoresistlayer 15 of the photomask blank by using the exposure system 400 (S33).

According to an exemplary embodiment of the inventive concept, the loweroptical system 200 and the exposure system 400 may be aligned in asimilar or modified manner as illustrated with reference to FIGS. 5, 6,and 9. Similarly, the step S32 of aligning the lower optical system 200with the fiducial mark(s) FM may be performed in a similar or modifiedmanner as illustrated with reference to FIGS. 5 and 7.

The step S33 of irradiating the light source may include selectivelyirradiating the light source onto a predetermined region of thephotoresist layer 15 based on the defect avoidance data D3, asillustrated with reference to FIG. 4. For the purpose of the selectiveirradiation, the position of the stage ST may be adjusted such that theexposure system 400 is fixed during the step S33 of irradiating thelight source.

FIGS. 11 and 12 are diagrams illustrating the alignment of the exposuresystem 400 and the lower optical system 200 according to an exemplaryembodiment of the inventive concept.

Referring to FIGS. 11 and 12, the alignment of the lower optical system200 and the exposure system 400 may be achieved by using an auxiliaryoptical system having a relative position fixed with respect to theexposure system 400. For example, an auxiliary optical system mayinclude a mirror 410 inclined at an angle of approximately 45° and analignment optical system 420, which may be disposed on one side of theexposure system 400, as illustrated in FIG. 11. According to anotherexemplary embodiment of the inventive concept, an auxiliary opticalsystem may include a flat mirror 430 disposed on one side of theexposure system 400, as illustrated in FIG. 12.

According to the exemplary embodiment of the inventive conceptillustrated in FIG. 11, the alignment optical system 420 aligns themirror 410, which is inclined at an angle of approximately 45°, with thelower optical system 200. The optical axes of the exposure system 400and the lower optical system 200 may be effectively aligned. Forexample, since the relative position r₀ between the mirror 410, which isinclined at an angle of approximately 45°, and the exposure system 400is fixed, the location of the fiducial mark FM identified by the loweroptical system 200 may be effectively used as a reference point whenexposing the photomask blank to a light source by the exposure system400 (Step S33). Similarly, according to the exemplary embodiment of theinventive concept illustrated in FIG. 12, the lower optical system 200may be aligned with the exposure system 400 in an off-axis manner byusing the flat mirror 430 without a separate system alignment pattern150.

According to another exemplary embodiment of the inventive concept, theoff-axis alignment using the auxiliary optical system may be used whenaligning the lower optical system 200 with the upper optical system 300.

FIGS. 14 and 15 are diagrams illustrating the stage according to anexemplary embodiment of the inventive concept.

As illustrated in FIG. 6, at least one opening O is provided in thestage ST on which the photomask blank PB is disposed. The opening Oexposes the fiducial mark(s) FM for alignment with the lower opticalsystem 200. According to another exemplary embodiment of the inventiveconcept, the stage ST may include an opening substantially exposing theentire lower surface of the photomask blank PB, as illustrated in FIG.14. According to another exemplary embodiment of the inventive concept,in order to prevent the photomask blank PB from bending, the stage STmay further include a support member SP supporting the lower surface ofthe photomask blank PB, as illustrated in FIG. 15.

According to the exemplary embodiments of the inventive concept, thefiducial mark(s) FM may be formed on the lower surface of the photomask,allowing the defect avoidance technique, as described above, to beapplied. As a result, location information, which is generated when thethin layers are formed on the upper surface of the photomask substrate,may be measured and recorded with high accuracy

Although the present invention has been described in connection with theexemplary embodiments of the present invention illustrated in theaccompanying drawings, it is not limited thereto. It will be apparent tothose skilled in the art that various substitution, modifications and/orchanges may be applied thereto without departing from the scope andspirit of the invention.

1. A reflective photomask comprising: a photomask substrate; a photomaskpattern, formed on an upper surface of the photomask substrate; at leastone alignment mark, formed on the upper surface of the photomasksubstrate, for aligning the reflective photomask with an exposureapparatus; and at least one fiducial mark, formed on a lower surface ofthe photomask substrate, for determining locations of defects in thephotomask pattern.
 2. The reflective photomask of claim 1, wherein thephotomask pattern comprises a plurality of thin layers.
 3. Thereflective photomask of claim 2, wherein the plurality of thin layerscomprises at least one of a multilayer, a capping layer, a buffer layer,an absorbing layer, and a photoresist layer.
 4. The reflective photomaskof claim 2, wherein the plurality of thin layers comprises alternatingmolybdenum layers and silicon layers.
 5. The reflective photomask ofclaim 1, wherein the photomask pattern and the alignment mark are formedon a top layer of the upper surface and the fiducial mark is formed on abottom layer of the lower surface.
 6. The reflective photomask of claim1, wherein the photomask substrate comprises a material having a lowthermal expansion property.
 7. The reflective photomask of claim 6,wherein the material comprises glass.
 8. The reflective photomask ofclaim 1, further comprising a conductive layer for fixing the reflectivephotomask to an exposure apparatus via an electrostatic chucking effect.9. The reflective photomask of claim 2, wherein the plurality of thinlayers forms a Bragg reflector. 10-19. (canceled)